Project Summary/Abstract. ?-lactam antibiotics are the most widely used antibiotic class in the U.S., accounting for more than 50% of antibacterial prescriptions. Among other roles, they are critical therapeutics for difficult-to-treat infections due to gram negative pathogens such as E. coli, Klebsiella pneumoniae, and Enterobacter spp. However, the utility of this class of antibiotics is being rapidly compromised by the alarming spread of new ?-lactamase resistance mechanisms; enzymes produced by bacteria that hydrolytically inactivate ?-lactam antibiotics. A particularly important concern is the lack of orally bioavailable ?-lactam/?-lactamase inhibitor (BLI) combinations capable of addressing this emerging challenge, both in the Biodefense arena and in the hospital/community settings. Orally available ?-lactam combinations with legacy BLIs (e.g., amoxicillin/clavulanic acid or Augmentin) demonstrate reasonable activity against Gram negative pathogens expressing Ambler Class A Extended Spectrum Beta Lactamases (ESBLs), but lack activity against organisms expressing Class A carbapenemases (KPC-type), Class C cephalosporinases (chromosomal and plasmidic) and Class D oxacillinases. We have identified a compound series with potent and broad spectrum activity against these serine ?-lactamases. These compounds rescue the activity of the orally bioavailable cephalosporin cefixime in MDR-strains of Gram negative Enterobacteriaceae, including E. coli, and K. pneumoniae. Moreover, we have shown that prototype prodrugs in the series demonstrate striking oral bioavailability in rodents, and that they rescue cefixime activity in murine models of bacterial disease. The objectives of this project are to advance the selected Development Candidate to IND filing for use in combination with cefixime.